The present invention relates to a variable conical fillet between an airfoil tip of a turbine bucket and a bucket tip shroud and particularly relates to a conical fillet shaped and sized to improve part life, performance and manufacturing of the turbine bucket.
Turbine buckets generally comprise an airfoil, a platform, shank and dovetail along a radial inner end portion of the bucket and often a tip shroud at the tip of the airfoil in mechanical engagement with tip shrouds of adjacent buckets. The tip shroud and airfoil of a turbine bucket are typically provided with a simple fillet shape of a predetermined size and generally of a constant radius about the intersection of the tip shroud and the airfoil tip. That is, a generally uniform radius was applied to the shroud fillet as the fillet was applied about the intersection of the airfoil tip and tip shroud. The fillet lowered the stress concentration between the airfoil and tip shroud.
While the stresses were reduced by use of constant radius fillets, that fillet design inefficiently distributed mass and resulted in poorly balanced stresses. High stresses were localized at various locations or points in and about the fillet between the airfoil and tip shroud and such localized high stresses lead to significant decreases in bucket life. Thus, while stresses were reduced by the application of fillets of constant radius, the localized high stresses in critical areas were still present. These stresses reduced the creep life of the tip shroud which can lead to premature failure of the bucket. Additionally, tip shroud-to-tip shroud engagement was sometimes lost, with resulting shingling of the tip shrouds. It will also be appreciated that the failure of a single bucket causes the turbine to be taken offline for repair. This is a time-consuming and costly outage, causing the customer as well as the turbine producer to incur higher costs due to unproductivity, labor, part repair, outage time and replacement. Consequently, there has developed a need for a customization of the fillet between the tip of the airfoil and the tip shroud of a bucket to provide a more uniform distribution of stress taking into account the high localized stresses about the fillet as well as reducing the mass of the fillet thereby to extend the creep life of the tip shroud.